Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism.
FIG. 1B is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A.
Arrow 14 represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency 10 of a base unit transmitter having a value of F.sub.BASE, which is located near by. In the present example the mobile unit further detects a carrier frequency 12 provided by a neighbor transmitter, having a value of F.sub.NEIGHBOR. The value of the mobile unit F.sub.MOBILE is located between the values of the base unit frequency F.sub.BASE and the neighbor mobile transmitter frequency F.sub.NEIGHBOR.
In the present example the mobile unit 14 detects the signals provided by base 10 and the neighbor 12 wherein the received power of the neighbor 12 is higher than the received power of the base unit 10.
According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency 12.
It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc 16. These techniques often fail when the power of the undesired signal is significantly high.
Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency F.sub.NEIGHBOR and starts progressing its frequency 14 towards F.sub.NEIGHBOR. Finally the synchronization mechanism allows the frequency of the mobile unit 14 to acquire and synchronize with the frequency of the neighbor unit 12. This is shown in FIG. 1B by aligning line 12 and arrow 14. As can be seen, at this stage the frequency 10 of the base transmitter is filtered out by the filter 16.
A conventional synchronization mechanism provides frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that the F.sub.NEIGHBOR can be located outside this range. in such a case, F.sub.MOBILE, might get stuck at the boundary frequency value which is closest to F.sub.NEIGHBOR.
It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit 14 is synchronized with the frequency of neighbor unit 12 instead of the frequency of the base unit 10, is not acceptable.